The present invention relates to an image signal processing. In particular, it relates to an image signal processing method and an image signal processing apparatus for processing color information of an image signal.
In image processing, which involves an enormous amount of data, image signals can be compressed for processing. For example, digital image signals can be represented by Y (luminance), Cr (chrominance (red-yellow)) and Cb (chrominance (blue-yellow)) according to three different formats: YCrCb 4:4:4, YCrCb 4:2:2 and YCrCb 4:2:0, which have less color information and higher compression rates in this order.
According to the YCrCb 4:4:4 format, every pixel has a Y value, which is luminance data, and a Cr value and a Cb value, which are chrominance data.
According to the YCrCb 4:2:2 format, each pair of adjacent pixels in the horizontal direction have individual Y values, which are luminance data, but share a Cr value and a Cb value, which are chrominance data.
According to the YCrCb 4:2:0 format, the YCrCb 4:2:2 format is expanded to pairs of adjacent lines in the vertical direction. That is, in a pair of adjacent lines, a pair of adjacent pixels in the horizontal direction has individual Y values, which are luminance data, and not only the pair of adjacent pixels in the horizontal direction but also a pair of pixels in the adjacent lines shares a Cr value and a Cb value, which are chrominance data. Typically, the Cr value and the Cb value, which are chrominance data on the pixels in the upper line, are applied to the pixels in the lower line of the adjacent line pair.
Conversion of a YCrCb 4:2:2 image signal into a YCrCb 4:2:0 image signal involves thinning out the color information and therefore makes it impossible to display an image of a thin line having a one-pixel width. For example, consider the case where one horizontal red line (having a one-pixel width) is drawn on a black screen, and the screen is converted from the YCrCb 4:2:2 format to the YCrCb 4:2:0 format. If the line representing the horizontal line is a line that retains the red color information without being thinned out, the retained red color information is encoded, so that no problem arises. However, if the line representing the horizontal line is a line that retains no red color information because of thinning out, the color information on the line above, which is black, not red, because the screen is black, is applied to the line. Consequently, the thin line loses the red color information and appears gray.
FIG. 1 shows YCrCb 4:2:2 image signals and YCrCb 4:2:2 image signals for illustrating how a thin line loses red color information. FIG. 1(A) shows YCrCb 4:2:2 image signals for pixels (0, 0) to (1, 3) in two adjacent lines 0 and 1 in the vertical direction. On the assumption that each of the Y value, which is luminance data, and the Cr value and the Cb value, which are chrominance data, is 8-bit data, and the lines 0 and 1 appear black and red, respectively, the pixels (0, 0) to (0, 3) have individual Y values (Y=0 in this example), which are luminance data, and each pair of adjacent pixels in the horizontal direction share a Cr value (Cr=128 in this example) and a Cb value (Cb=128 in this example), which are chrominance data. The pixels (1, 0) to (1, 3) have individual Y values (Y=76 in this example), which are luminance data, and each pair of adjacent pixels in the horizontal direction share a Cr value (Cr=255 in this example) and a Cb value (Cb=85 in this example), which are chrominance data.
FIG. 1(B) shows YCrCb 4:2:0 image signals for pixels (0, 0) to (1, 3) in the lines 0 and 1 resulting from the compression. The pixels (0, 0) to (0, 3) in the line 0 have the same Y value (Y=0), which is luminance data, and the same Cr value (Cr=128) and the same Cb value (Cb=128). However, the pixels (1, 0) to (1, 3) in the line 1 have different chrominance data, a Cr value of 128 and a Cb value of 128, which are the chrominance data of the pixels (0, 0) to (0, 3) in the line 0, although the Y value, which is luminance data, has not changed (Y=76). In this way, the pixels (1, 0) to (1, 3) in the line 1 loses the red color information and retain only the Y value (Y=76), which is luminance data, and therefore appears as a gray-scale line.
As shown in FIG. 2, according to prior art, such a thin line becomes a gray-scale line and thus less visible or, if the line is oblique, appears as a dashed line. Conventionally, it was a common practice to ignore the loss of color information in the course of the compression to the YCrCb 4:2:0 format and avoid rendering such a very thin line segment, and there is no substantial solution.
Patent Literature 1 discloses a signal converting circuit in a digital camera that performs a conversion from the YCrCb 4:2:2 format to the YCrCb 4:4:4 format.
Patent Literature 2 discloses an electronic device that accepts the YUV422 format and YUV420 format in a color space coding framework.
Patent Literature 3 discloses a technique of recording digital television signals in a recording medium by converting input YCrCb 4:2:2 component signals into YCrCb 4:2:0 component signals through a line sequential processing, in which two chrominance signals Cr and Cb, which have less visual effects, are thinned out at a rate of one line per field, and then performing a compression coding processing.
Patent Literature 4 discloses a video conversion apparatus that has an interlace/progressive converter that converts the video format of a video signal from the interlace format to the progressive format and a chrominance format converter that converts the chrominance format of a video signal from the 4:2:0 format to the 4:2:2 format. The video conversion apparatus can reduce deterioration of the quality of an image represented by a video signal by the interlace/progressive converter converting a received interlaced 4:2:0 video signal by performing interpolation of the luminance component and the chrominance components of the video signal and the chrominance format converter converting the video format before converting the chrominance format of the video signal.